The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Distributing electric power in three phases is a common method of electric power transmission. A three-phase power system uses three alternating current (AC) voltage sources whose phases are each separated by 120 degrees. In a balanced three-phase power system, three power supply lines carry three AC signals of the same frequency (and therefore the same period), which reach their instantaneous peak values at different times. Taking a current carried by one of the three power supply lines as a reference, the other two currents are delayed in time by one-third and two-thirds, respectively, of one period of the electrical current.
One common way to produce the three AC voltage sources is to construct an AC generator/alternator in which a rotating magnetic field passes by three sets of wire windings, each set spaced 120 degrees apart around the circumference of the generator/alternator. A phase voltage refers to the amount of voltage measured across any one AC voltage source, such as one wire winding set in an AC generator/alternator. Three-phase power may be used to power motors and many other devices. For example, three-phase power may be used to power hermetic compressor motors such as motors in reciprocating compressors.
The windings of a motor can be connected to three-phase power supply lines in different configurations, including those shown in FIGS. 1A and 1B. More specifically, FIG. 1A depicts a “Δ” (delta) configuration, while FIG. 1B depicts a “Y” configuration, also known as a star configuration. In the delta configuration, the windings are connected between the three power supply lines. In the star configuration, the windings are connected from each of the three power supply lines to a common node. The common node may be connected to a neutral line (N), which is present in some applications of the star configuration. The neutral line N may allow lower voltage devices to be connected between one of the power supply lines and the neutral line, which provides a lower voltage than when connecting between two of the power supply lines.